We propose to continue our investigations into the mechanism of action of replication terminator proteins of the yeasts S. cerevisiae and the distantly related fission yeast Schizosaccharomyces pombe by using a combination of yeast genetics, in vivo and in vitro biochemistry and to start work towards the longer term goals of setting up crystallization trials of the terminator proteins bound to the terminus DMA. In S. cerevisiae we wish to uncover the mechanism of action of the Fob1 terminator protein by performing in vitro biochemical analysis to investigate if the protein is a contrahelicase that antagonizes the activities of eukaryotic replicative helicases. We also propose to work out the mechanism of action of the checkpoint proteins Tofl and Csm3 at the replication termini. We have discovered that the spacer region of rDNA of S. pombe has four not three replication arrest sites. Two of the four sites require the interesting protein Reb1 p that not only arrests forks in a polar mode but also terminates the transcription of rDNA and probably also acts as a transcription factor that turns on transcription. We wish to work out the mechanism of action of Reb1p. We have narrowed down the location of Ter1 of S. pombe to ~30bp and we wish to clone the gene that encodes the cognate terminator protein and investigate its mode of action. Finally, we wish to uncover the mode of action of the Swi1 and Swi3 proteins to determine if these act as "antisweepases".